PRETORIA, South Africa — Stifling sobs, Oscar Pistorius took the witness stand Monday in his murder trial and apologized to the family of the girlfriend he shot dead, describing himself as being traumatized and awakening from nightmares to the “smell of blood.”

Pistorius’ voice quavered so much and was so low that Judge Thokozile Masipa asked him to speak up to the packed courtroom as he described his remorse for having killed Reeva Steenkamp on Feb. 14, 2013. He said he mistook her for an intruder when he fired four times through a locked toilet stall door in his home. Prosecutors said he shot her as she screamed in terror after they had an argument in the predawn hours of Valentine’s Day.

“There hasn’t been a moment since this tragedy happened that I haven’t thought about your family,” the double-amputee runner said as he addressed the courtroom and Steenkamp’s mother, June, looked straight at him, stone-faced.

“I wake up every morning and you’re the first people I think of, the first people I pray for … I was simply trying to protect Reeva. I can promise that when she went to bed that night she felt loved,” said Pistorius, stating the central part of his defense, that he shot Steenkamp by mistake, thinking she was an intruder in his bathroom.

Pistorius killed Steenkamp last year by shooting her in the head, arm and hip through a toilet door in his home. Prosecutors allege he intentionally killed her after a fight in the early hours of Feb. 14, 2013 and have sought to paint him as a hothead with an inflated sense of entitlement and an obsession with firearms.

In his testimony, Pistorius also said he is on antidepressant medicine and has trouble sleeping, and described one night when he went to hide in a closet after waking up in “a panic.”

“I climbed into a cupboard and I phoned my sister to come and sit by me, which she did for a while,” Pistorius said.

His testimony came on the day his defense opened its case and after legal experts said it was crucial to his case that he testify to explain why he shot Steenkamp. Pistorius faces 25 years to life in prison if convicted of premeditated murder for Steenkamp’s killing.

He spoke in a soft, quavering voice while making his apology and describing what he said was his fragile state. At one point, Judge Thokozile Masipa asked him to speak more loudly.

Later, Pistorius grew more settled and confident as defense lawyer Barry Roux led him through questions about his childhood, his athletics career and how he overcame his disability to run at top track meets. Pistorius’s life story is one that impressed many people around the world, before he killed Steenkamp.

Pistorius also described the positive affect his mother Sheila has on his life after he was born with a congenital condition and had to have his lower legs amputated when he was 11 months old, and the grief he felt at her death when he was a teenager.

But he also described how as a family they had “security concerns” and his mother slept with a gun under a pillow on her bed.

He was asked by Roux to talk about a 2009 boat crash when he suffered serious facial injuries. He said the accident had a “massive impact,” causing him to become fearful, withdrawn, more vigilant about personal safety and more focused on his sporting career. He denied media reports at the time that he had been drinking alcohol before the boat accident.